Continuous diesel soot control with minimal back pressure penatly using conventional flow substrates and active direct soot oxidation catalyst disposed thereon

ABSTRACT

There is disclosed high cell density or tortuous/turbulent flow through monolithic catalyst devices for the direct catalytic, and (semi) continuous oxidation of diesel particulate matter. The catalysts relate to OIC/OS materials having a stable cubic crystal structure, and most especially to promoted OIC/OS wherein the promotion is achieved by the post-synthetic introduction of non-precious metals via a basic (alkaline) exchange process. The catalyst may additionally be promoted by the introduction of Precious Group Metals.

REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional application61/308,879, filed Mar. 27, 2008, and is a continuation-in-part ofapplication Ser. No. 12/240,170 filed Sep. 29, 2008, and applicationSer. Nos. 12/363,310 and 12/363,329, both filed Jan. 30, 2009, all ofwhich are relied on and incorporated herein by reference.

INTRODUCTION AND BACKGROUND

Over the last thirty years increasingly stringent legislative limitshave been introduced to regulate the emissions from both petrol(gasoline) and diesel internal combustion engines. See Regulation (EC)No 715/2007 of the European Parliament and of the Council, 20 Jun. 2007,Official Journal of the European Union L 171/1, see also Twigg, AppliedCatalysis B, vol. 70 p 2-25 and R. M. Heck, R. J. Farrauto AppliedCatalysis A vol. 221, (2001), p 443-457 and references therein. In thecase of diesel/compression ignition engines this has led to theimplementation of the Diesel Oxidation Catalyst (DOC), Diesel NOxTrap/NOx Storage Catalyst (DNT/NSC) and Selective Catalytic Reductioncatalyst (SCR) to address gaseous emissions of CO, HC (DOC) and nitrogenoxides (NOx). However, in addition to the gaseous components, the dieselexhaust stream also contains entrained solids, commonly referred to asparticulate matter or soot. This carbon-based material is a byproduct ofincomplete combustion and arises due to heterogeneity of the air-fuelmixture within the cylinder and presents a unique and specific challengewith regards to its control and conversion into environmentally benignproducts. Thus, while previously it has been possible to meet all legalrequirements for exhaust emissions of particulates via engine-relatedcontrol measures only (SAE Paper 2007-01-0234, Pfeiffer et al.), thestringent targets embodied in, for example, Euro 5 or Euro 6 (Regulation(EC) No 715/2007 of the European Parliament and of the Council, 20 June2007, Official Journal of the European Union L 171/1) necessitates theintroduction of the Diesel Particulate Filter (DPF), aka the ‘Wall-FlowFilter’ to enable specific remediation of soot.

The DPF typically comprises an inert porous ceramic e.g. siliconcarbide, cordierite etc. monolith substrate which may be additionallywash-coated with an active catalytic formulation to facilitate thechemistries required of the device e.g. soot combustion, (secondary)emission control, NOx abatement, etc. The wash-coat formulation itselfis typically a heterogeneous-phase catalyst and may contain particles ofhighly active precious group metal (PGM) dispersed and stabilized on arefractory oxide support or supports; e.g. alumina. The DPF mayadditionally contain an Oxygen Storage (OS) component to enhance theregeneration function of the filter.

The DPF achieves high filtration efficiency of particulates as a resultof the physical filtration achieved by forcing the exhaust flow throughthe porous wall of filter. However, over time this results in a build upof stored material, commonly referred to as a filter-cake, within thefilter which results in an ever increasing back pressure penalty,arising from the work required to force the gas flow through anincreasingly dense flow restriction. This flow restriction leads to anunacceptable decrease in engine performance and hence, the filter-cakemust be combusted in order to ‘regenerate’ the filter to a near pristinecondition such that it is able to again store the carbonaceousparticulates with minimal back pressure penalty. However, at this time afully passive and continuous soot regeneration technology has not beendemonstrated on a vehicle and hence the complete regeneration of thefilter requires an “active” or forced regeneration strategy; see e.g.U.S. Pat. No. 7,441,403; U.S. Pat. No. 7,313,913. These activestrategies are reliant upon the manipulation of the gross reactionconditions of the exhaust in order to achieve filter regeneration.Hence, the regeneration of the particulate filter described above may beachieved by the use of auxiliary devices. For example, an air fuelnozzle and an ignition device can be used and operated, when desired, toheat the exhaust gases and the particulate filter to the temperaturerequired for the homogeneous combustion of the trapped particulatematter. In this manner, the trapped soot may be burned from the filtersurfaces to permit a continuous flow of the exhaust gases.Alternatively, an electric heater may be employed to generate the heatto initiate the combustion cycle; e.g. U.S. Pat. No. 7,469,532. Morecommonly however, the filter is regenerated by a so-called“post-injection” cycle in which secondary fuel is introduced, either bylate cylinder injection or via dedicated fuel injection unit in theexhaust train, and the hydrocarbons thus entrained in the exhaust floware combusted over an oxidation catalyst situated prior to the DPF togenerate a transient thermal ‘bloom’ within the filter which initiatesthe conversion of the soot into environmentally benign products (CO₂,H₂O); e.g. see SAE paper 2008-01-0481 and references therein.

However, the use of wall-flow filters to achieve efficiency inparticulate removal presents some immediate issues. Firstly due to thewall flow mechanism of filtration, the DPF introduces a significantback-pressure on the engine. Moreover, the addition of a washcoat to thebare filter increases this back pressure and the sieving action employedto trap soot results in yet a further and continuous increase in backpressure. As indicated previously any increases in backpressure are atthe expense of engine efficiency and lead to an ever increasing fueleconomy penalty, due to the wasted work pushing exhaust gas through thesoot filter cake, washcoat formulation and filter. Thus, significantefforts have been expended in the development of mechanically andthermally robust DPFs with high filtration efficiency but decreasedback-pressure penalty and in the development of active washcoatformulations capable of high conversions at minimal wash-coat loads inattempts to minimize the back-pressure/fuel economy issues otherwiseevident.

Additionally, there remain outstanding questions regarding theregeneration cycle employed by the DPF. Such traditional ‘active’ cyclesare all energy intensive and result in a substantial and unattractivefuel penalty; i.e. an additional and ongoing operational cost. Thus, theuse of sacrificial hydrocarbon species in the active regeneration cycleimposes as high as a 5% decrease in fuel economy. Moreover, theimplementation of an active emissions control strategy requires complexand accurate engine management protocols to avoid incompleteregeneration and/or untreated emissions; e.g. U.S. Pat. No. 7,412,822.In addition, soot combustion initiated in this manner results in aphenomenon known as ‘oil dilution’ which can both adversely affectengine operation and result in ash deposition (inorganic salts) withinthe filter which impact the back pressure, soot capacity and catalyticperformance of the filter; e.g. U.S. Pat. No. 7,433,776. Finally, itshould be noted that soot combustion initiated in this manner proceedsin a more homogeneous; i.e. non-catalytic manner and can beuncontrolled. This in turn can result in localized exothermic ‘hotspots’of extreme temperature (T>1000° C.) which can easily damage thecatalytic efficiency of a formulation (PGM sintering, PGM de-alloying,surface area and porosity collapse of the support oxide). In the worstcase scenario, catastrophic uncontrolled combustion of soot can destroythe DPF monolith through thermal degradation or even melting themonolith.

Hence, many attempts have been made to address or limit the extent ofthe issues related to the active regeneration strategy. Such efforts areexemplified by attempts to introduce more passive regenerationstrategies based upon the use of the redox chemistry of advanced OSmaterials, e.g. US 2005/0282698 A1. In these studies it was shown thatdecreases in the temperature required for soot oxidation may be achievedby utilisation of active oxygen species derived from a redox activewashcoat OS material. The OS materials used in the DPF are typicallybased upon CeO₂ or other redox oxide and are employed to ‘buffer’ thecatalyst from local variations in the air/fuel ratio during regenerationor other transient event. They do this by ‘releasing’ active oxygen fromtheir 3-D structure in a rapid and reproducible manner underoxygen-depleted transients, ‘regenerating’ this lost oxygen byadsorption from the gaseous phase when oxygen-rich conditions arise.This activity is attributed to the redox activity of CeO₂ via the2Ce⁴⁺→2Ce³⁺+[O²⁻] reaction. This high availability of oxygen is criticalfor the promotion of generic oxidation/reduction chemistries e.g. CO/NOchemistry for the petrol (gasoline) three-way catalyst, or more recentlyfor the direct catalytic oxidation of particulate, matter (soot) in theCDPF e.g. US 2005/0282698 A1, SAE 2008-01-0481 and references therein.This work is one of many studies examining the chemistry, synthesis,modification and optimisation of Ce—Zr based OS materials. For example,the use of Ceria-Zirconia materials doped with lower valent ions foremission control applications have been extensively studied e.g. U.S.Pat. No. 6,468,941, U.S. Pat. No. 6,585,944. These studies demonstratethat lower valent dopant ions such as Rare Earth metals, e.g. Y, La, Nd,Pr, etc., Transition metals e.g. Fe, Co, Cu etc. or Alkaline Earthmetals e.g. Sr, Ca and Mg can all have a beneficial impact upon oxygenion conductivity This is proposed to arise from the formation of oxygenvacancies within the preferred cubic fluorite lattice of the solidsolution which lowers the energy barrier to oxygen ion transport fromthe crystal bulk to the surface thereby enhancing the ability of thesolid solution to buffer the air fuel transients occurring in theexhaust stream of a typical petrol (gasoline) three-way catalystapplication.

Additionally it has been shown (U.S. Pat. No. 6,468,941 and U.S. Pat.No. 6,585,944) that the use of specific examples of the above dopantscan provide full stabilization of the preferred cubic fluorite latticestructure for ceria-zirconia solid solutions, with Y being identified ashaving particular benefit hereto. The presence of the preferred cubicfluorite structure has been found to correlate with the most facileredox chemistry for Ce⁴⁺→Ce³⁺, from both the surface and bulk of thecrystal, thus dramatically increasing the oxygen storage and releasecapacity, as compared to bulk CeO₂. This benefit is especiallypronounced as the material undergoes crystal growth/sintering due to thehydrothermal extremes present in typical exhaust environments. Theincorporation of especially Y and to a lesser extent La and Pr have alsobeen demonstrated to limit or, in certain cases, circumvent thedisproportionation of the single cubic phase Ceria-Zirconia into acomposite consisting of more Ce-rich cubic phases and more Zr-richtetragonal phases, a process which results in marked decrease in redoxfunction, surface area etc. of the solid solution.

Finally, U.S. Pat. No. 6,468,941, U.S. Pat. No. 6,585,944, U.S. patentapplication Ser. No. 12/363,310 and U.S. patent application Ser. No.12/363,329 (both applications being incorporated herein by reference)teach the potential for employing base, i.e. non-precious group (Pt, Pd,Rh, Au etc.) dopant metals into or with the cubic fluorite lattice ofthe solid solution as an alternative means to promote the redoxchemistry of cerium, with Fe, Ni, Co, Cu, Ag, Mn, Bi and mixtures ofthese elements being identified as of particular interest. Hence whiletypical non-promoted OS materials typically exhibit a redox maximum, asdetermined by H₂ Temperature Programmed Reduction (TPR), at ca. 600° C.,the inclusion of base metals within the lattice can decrease thistemperature by >200° C. at a fraction of the cost incurred by the use ofprecious metals.

However, while these base metals can be beneficially incorporated in theCeZrOx lattice and this incorporation can significantly promote lowtemperature redox function for fresh materials, the addition of theseelements can also decrease fresh and aged phase purity and significantlydecrease hydrothermal durability (promote crystal sintering and materialdensification), leading to losses in aged performance cf. basecompositions without additional base metal. In addition, duringconventional aging cycles reactions may occur between the gas phase andthe CeZr material which can result in extraction of these additionalbase elements from the cubic fluorite lattice. This, in turn, can resultin formation of separate bulk phase(s) with low intrinsic catalyticactivity or in a worst case scenario, phases which directly interactwith the OS or other catalyst component resulting in a direct orindirect poisoning of the catalyst. Hence, until recently, particularsynthetic care was required to enable the incorporation of promotantlower valent ions into the cubic fluorite structure while ensuring boththe electrical neutrality and phase preservation. Thus, as shown in U.S.application Ser. No. 12/363,310, the synthesis of an OS materialcontaining a specific low valent base metal promoter (Ag) ‘doped’ into acubic fluorite structure with ca. 40% Ce resulted in phasedisproportionation into Ce-rich and Ce-poor domains, with a markeddecrease in redox performance. This contrasted with a newly developedbasic exchange process which was able to provide an equivalentcomposition with high activity and hydrothermal durability for use inthe DPF.

Unfortunately, despite the large number of attempts to employ advancedOS materials in either passive or active regeneration methodologies invehicular applications these have previously met with limited success.Extensive studies of the chemistry occurring in these systems havedemonstrated that the activity of the OS-based catalyst is dependentupon high ‘Contact Efficiency’ between the OS material and the soot;e.g. see, Applied Catalysis B. Environmental 8, 57, (1996). Subsequentstudies, described in SAE paper 2008-01-0481 and U.S. application Ser.Nos. 12/363,310 and 12/363,329 have now identified that the loss ofcontact efficiency between the OS and soot may arise from specificchemistries involving the significant NO engine emissions typical ofpre-EuroV legislation engines. This process has been denoted as‘de-coupling’ of the OS and soot and is the result of the reaction ofengine out NO over oxidized PGM to produce NO₂ which combusts the sootin the immediate environment of the catalyst producing CO+NO. The NObyproduct of this process is further ‘recycled’ to NO₂ and the sootcombustion re-initiated, again removing only that soot which immediatelycontacts the catalyst. This cycle is the basis of U.S. Pat. No.4,902,487 and was previously believed to be the major reaction providinglow temperature soot combustion/regeneration. However, this mechanismappears only effective at removing low concentrations of soot and indeedonly that proportion of soot in direct contact with the catalyst. Thus,this mechanism effectively ‘de-couples’ the catalyst and soot anddramatically decreases the effectiveness of the OS-mediated regenerationmethod and may in fact be considered to be a reactive poison whicheffectively ‘deactivates’ the ‘true’ OS mediated low temperature,passive, soot regeneration reaction required for optimum soot emissioncontrol. Fortunately, however, the design of the next generation ionexchanged OS materials has been found to be effective at bothcircumventing this ‘de-coupling’ process and also in promotion of theredox characteristics of the OS and hence demonstrated robustperformance benefits with respect to soot regeneration on all both theengine dynamometer and in vehicle trials on wash-coated DPFs (see U.S.application Ser. No. 12/363,329).

Based upon the aforementioned requirements and challenges, it isapparent that the conventional approach of using the wall-flow DPF toensure highly effective trapping and subsequent combustion ofparticulates presents many challenging technical obstacles. Accordingly,what is needed in the art are improved materials and/or methods for thecontrol and conversion of particulate matter whilst offering both areduction in fuel penalty for regeneration but also decreased complexitywith regards to initiation and control of any regeneration cyclecompared to the traditional DPF and conventional active regenerationstrategy. Herein we propose the use of an in-line soot combustion deviceand catalyst with minimal/significantly decreased back-pressure penaltyfor the (semi) continuous and (semi) passive combustion of retainedparticulate matter at significantly lower temperatures than thoserequired for the conventional wall-flow filter.

SUMMARY OF THE INVENTION

A significant advance in the development of a method and apparatus forthe (semi) continuous, direct catalytic, oxidation of diesel particulatematter may be realised by the combination of base metal modified OxygenStorage (OS) materials with a conventional flow substrate. The substrateis selected from a range of ceramic or metallic technologies upon whichthe active washcoat is disposed. Such substrates can be metallic parts,ceramic or metal foams. The substrate is further characterised bypresenting a high number of channels or cells per unit area or by theability to introduce turbulent flow due to the construction of itsinternal flow channels. The particular combination of the base metalmodified OS direct soot oxidation catalyst with the flow throughmonolith provides a synergy which enables high conversion of particulatematter without the backpressure penalty introduced by the conventionalDPF. Specifically, the synergy is believed to arise from the ability ofthe active OS to combust soot at lower temperatures which in turn isfacilitated by the decreased thermal mass of the conventional substrate,with the latter still providing sufficient geometric surface area forsoot deposition and reaction. This provides for the large improvementsin lower temperature activity and is in marked contrast to theconventional wall flow DPF wherein large thermal mass of the substrate,particularly for SiC DPF, inhibits initiation and especially propagationof soot combustion. Thus, this combination of technologies provides ameans for the effective conversion of particulate matter underconditions more typical of the standard driving cycle i.e. sootcombustion without recourse to high temperature active regenerationcycles and the various penalties and other issues associated thereto.

The doped OS materials herein are based upon ZrO₂/CeO₂ solid solutionscontaining a substantially phase pure cubic fluorite structure and areproduced by the specific ion exchange of base i.e. non-precious groupmetals. The range of appropriate materials and full details regardingexecution of the ion exchange are described in U.S. application Ser.Nos. 12/363,310 and 12/363,329. The mode of ion exchange essentiallyinvolves the introduction of active metal/cations into the solidsolution under chemically basic, i.e. conditions of high pH, that is sayhigh OH⁻/low hydronium (H₃O⁺) or proton (H⁺) content. As demonstrated inthe aforementioned work, the resultant materials demonstrate highactivity and hydrothermal durability in contrast to any promotionrealized by conventional impregnation of an acidic metal e.g. metalnitrate, where formation of bulk oxide phases in fresh materials andrapid sintering of such oxide phases, with resultant deactivation, isthe norm. The proposed exchange of the H⁺ species, present at Ce³⁺defect sites within the Ce—ZrOx lattice, by metal ions enables theincorporation and stabilization of specific mono-valent e.g. K⁺,di-valent e.g. Cu²⁺, tri-valent e.g. Fe³⁺ and higher valence ions athigh dispersion within the oxide matrix. The choice of base metals thusincorporated is based upon oxides known to be active for reactions ofespecial interest or catalytic importance. Metals of specific catalyticsignificance include Ag, Cu, Co, Mn, Fe, alkali metals, alkaline earthmetals or transitions metals, or other metal or metalloid known to forma stable nitrate which can undergo subsequent decomposition andreduction N₂ under conditions within the conventional operational windowof the vehicle exhaust. The term “transition metal” refers to the 38elements in Groups 3 to 12 of the Periodic Table of Elements.

The use of high cell density/turbulent flow through monoliths is alsorequired to provide sufficient interaction and subsequent reactionbetween the entrained soot particles within the exhaust flow and theactive catalytic coating. The term high cell density is consistent withpreformed flow through monolith substrates with a large (≧600) number ofindividual cells of flow channels per square inch. It is proposed thatthis high cell density firstly introduces turbulence at the inlet tomaximise possible soot collisions with the active wash-coated walls ofthe monolith. Secondly, the high cell density restricts the flow paththrough the monolith, again increasing the potential for particulatecollisions and retention/reaction on the active wash-coat, but withoutthe large backpressure penalties associated with the conventional DPF.Moreover the use of the flow-through substrate removes existingconstraints regarding total washcoat loading, or the use of layeredtechnologies with specific functionalities, e.g. soot combustioncatalyst in one layer (overcoat) and SCR catalyst in a second layer(undercoat), equally it enables the use of an undercoat rich in Al₂O₃ toprovide high washcoat adhesion, but with low intrinsic catalyticfunction, onto which a second pass containing all required OS, PGM andNOx trap etc. active components may be dispersed. In this secondexample, the overcoat would under normal conditions present loweradhesion and would conventionally be diluted with binder, e.g. Al₂O₃,however, the incorporation of binder results in a decrease in activitydue to dilution of the active phase, hence the layered design ispreferred. This layering ensures the surface coating that wouldinteract/react with the soot as it passed through the flow-throughsubstrate would exclusively consist of active material and wouldtherefore maximize catalytic action. The enabling of higher washcoatloads when using the flow through monolith also provides the capabilityof employing higher concentrations of active materials to be coated onthe substrate thereby further enhancing the performance and hydrothermaldurability of the technology without the catastrophic back pressurepenalty such an approach would present using the conventional DPF. Henceby use of the flow through substrate washcoat load could be increasedfrom 10 g/l to 180 g/l or higher concomitantly increasing the effectivegeometric surface area for catalyst to soot contact to again increase incombustion efficiency.

In addition by use of the flow though monolith the texturalcharacteristics of the washcoat e.g. particle size, roughness etc. maybe optimised for activity rather than merely to minimize back pressurepenalty. Conventional formulations for DPFs typically target a D₅₀(diameter of particle at 50%) value of 5 microns or less to enable‘in-wall’ coating, i.e. coating of the internal porosity of thesubstrate without formation of a discrete washcoat layer on the surfaceof monolith, in order to minimize backpressure penalty. Such a particlesize distribution is typically achieved by aggressive milling of the rawmaterials used in the washcoat. However, the use of this ‘hyper-milling’to obtain the very small particles for in-wall coating has been found tobe extremely destructive to the activity, stability and surface areas ofthe OS and alumina components employed in typical formulations. As aresult such a process can adversely affect the rate of release and totalOxygen Storage capacity of the OS. In addition the hyper-milling canresult in cation extraction and phase disproportionation for the OS withfurther poisoning of any PGM function arising from the deposition ofextracted cations. In contrast, the use of a washcoat with hightextural/roughness characteristics has previously been identified asbeneficial in three-way applications (e.g. see SAE 2005-01-1111) and mayenhance initial flow turbulence and thus increase the probability ofcatalyst to soot contact. The retention of texture due to the absence ofaggressive milling can also be expected to increase the probability ofprimary soot moieties effectively contacting the OS material. The extentof intimate contact has been shown to correlate directly with directsoot combustion (see Applied Catalysis B. Environmental 8, 57, 1996,U.S. application Ser. No. 12/363,329, SAE 2008-01-0481). Moreover, theintegrity of the active formulation with respect to phase, OS functionor PGM functionalities is always of prime importance especially hereinsince it has been demonstrated that the energy produced by thecombustion of HC, CO or the SOF (soluble organic fraction) present insoot matter have been identified as a means of initiation andpropagation of the combustion of the remaining soot (akin to striking amatch or a primer, see SAE 2008-01-0481).

Benefits and features of the present invention include:

a) Provision of a hydrothermally robust direct soot catalyst system,active at temperatures relevant to diesel vehicle operation for (semi)continuous, direct catalytic, oxidation of soot;

b) Particulate control system without requirement for DPF substratethereby removing associated substrate cost, back pressure constraints,canning and space requirements and ancillary systems associated withconventional DPF;

c) Provision of an active catalyst providing full oxidation functionwithout recourse to complex conventional active regeneration cycles withassociated fuel penalty, filter cake formation, potential forcatastrophic uncontrolled regeneration, oil dilution, ash deposition orother issue associated with conventional DPF;

d) Flexibility of coated part design with respect to washcoat load,particle size/texture and hence the ability to optimize washcoat basedupon performance and durability requirements and not merely backpressureconstraints;

e) Ability to employ multilayer technologies with specificfunctionalised layers to providing additional catalytic properties andfunctions from a single monolith and to potentially achieve furtherchemical synergies and performance advantages previously impossible whenemploying the conventional DPF.

f) Synergistic operation between the active washcoat and high celldensity substrate to facilitate rapid oxidation of soot and solubleorganic fraction to thereby circumvent the potential for ‘faceplugging’, a phenomenon associated with the use of conventional highCPSI monoliths with conventional catalyst formulations.

This strategy clearly contrasts to those employed in conventional DPFsystems. For the conventional design catalytic functionality istypically more limited i.e. control of CO, HC from primary or secondaryemissions ([SAE Paper 2007-01-0234, Pfeiffer et al.), NH₃—SCR of NOx (US2008/202107-A), etc. Moreover, the design constraints for conventionalformulations are significant and are typically based upon the primarybalance required between filtration efficiency and maximum systembackpressure.

In order to meet the design targets for this synergistic operation ofcatalyst and monolith there are several key performance requirementsthat must be met. Firstly, there is a requirement for increased ceriareducibility at lower temperatures than is conventionally obtained withbinary, tertiary or even quaternary Ce—Zr—REOx systems. Thisreducibility is critical to achieve the low temperature O-ion donationfrom the catalyst to the soot which has been proposed as being a keyreaction step (SAE 2008-01-0481; U.S. application Ser. Nos. 12/363,310and 12/363,329; Appl. Catal. B vol. 17, 1998, p 205, Appl. Catal. B vol.75, 2007 p 189, Catal. Today 121, 2007, p 237, Appl. Catal. B vol. 80,2008, p 248]. Hence examinations of the use of CeOx or CeZrOx containingoxide solid solutions for soot oxidation have been widespread. Howeverconventional CeZrOx solid solutions, as typically employed in three-waycatalysts, typically exhibit a redox maximum, as determined by H₂Temperature Programmed Reduction (TPR) at ca. 600° C. This imposes therequirement for high exhaust gas/reaction temperatures in theapplication in order for the OS material to provide the maximum“buffering” or oxygen donation benefit. Moreover, this requirement forhigh temperature to access the active lattice oxygen is a barrier to theimplementation of CeZrOx for lower temperature direct soot oxidation. Inorder to address this temperature issue OS materials are typically“promoted” by the addition of a Precious Group Metal (PGM) component,e.g. Pt, Pd or Rh. However, promotion by these metals contributes alarge additional cost to the price of the emission control system.Moreover the addition of PGM, especially Pt, promotes the ‘classical’chemistry of NOx-mediated soot oxidation as described in U.S. Pat. No.4,902,487. However, it has now been found and clearly demonstrated thatNOx mediated soot oxidation is only effective at removing lowconcentrations of soot and indeed only that proportion of soot in directcontact with the catalyst and may be considered to be an effectivecatalyst poison for direct OS mediated soot oxidation (SAE 2008-01-0481,U.S. application Ser. Nos. 12/363,310 and 12/363,329). Thus, what isrequired in the art is a method to promote the oxygen ion conductivityof the CeOx/CeZrOx-based oxide material, but without use of expensivePGM and without the undesirable consequence of increasing the NOxoxidation chemistry of the catalyst.

A second limitation, again typical OS materials used to date, is alimitation with regard to their total Oxygen Storage Capacity, that isto say the amount of available oxygen as measured by TPR is typicallylower than that expected from consideration of the total Ce IV contentof the OS material. Many data available to date are consistent with aslittle as only ca. 50% of the total Ce IV available undergoingreduction. At this time it is uncertain whether this is due to afundamental issue, or due to limitations with the current syntheticmethod(s) employed in the manufacture of the OS material leading to amixed Ce IV/Ce III valency or whether a combination of additionalchemical, structural or textural limitations are responsible.

Finally, typical OS materials provide only limited, if any, additionalsynergies to the emission control system. As described elsewhere, idealmaterial components provide additional integrated chemical mechanisms tofurther enhance emissions control, e.g. NOx scavenging and reduction toN₂.

Hence, while OS materials are key components in realising highly activeand materials present significant limitations to development of the nextgeneration of exhaust catalyst that will be required to comply withnewer and ever more stringent emission targets. What is required is anew class of OS materials that are active at lower temperatures,especially the Cold Start portion of vehicular applications to promotecatalytic function. These OS materials should also display highhydrothermal durability and be tolerant to potential exhaust poisons inorder to enable their use in the wide range of demanding exhaustenvironments.

The above-described and other features will be appreciated andunderstood by those skilled in the art from the following detaileddescription, drawings, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood with reference to theaccompanying drawings, wherein:

FIG. 1—shows a schematic of the synthetic gas bench (SGB) reactor inwhich the concept trials were executed;

FIG. 2—shows the impact of reactive gas mixture during loading onsubsequent soot combustion;

FIG. 3—outlines the various gas compositions employed during the loadingand regeneration trials;

FIG. 4—compares the back pressure (hereafter B.P.) response for a 400CPSI (cells per square inch) flow through monolith during a three hour(10800 s) soot loading cycle as a function of either gas environmentduring loading or temperature and gas environment;

FIG. 5—impact of reaction conditions during loading (ex FIG. 4) onsubsequent soot combustion cycle via TPO;

FIG. 6—compares the O₂ concentrations at the reactor outlet during theTPO cycles described in FIG. 5;

FIG. 7—impact of monolith cell density on B.P. response during sootloading;

FIG. 8—impact of monolith cell density on combustion of retained soot;

FIG. 9—impact of cell density on O₂ consumption for combustion ofretained soot;

FIG. 10—impact of soot loading temperature for a 900 CPSI flow throughmonolith;

FIG. 11—displays the B.P. response and O₂ consumption traces associatedwith TPO cycles described in FIG. 10;

FIG. 12—shows an example of a temperature programmed soot loading usinga 900 CPSI monolith;

FIG. 13—shows a temperature programmed reaction experiment performedafter the temperature programmed reaction soot loading in FIG. 12;

FIG. 14—shows the TPO results for the 900 CPSI monolith after a sootloading with reactive gas and temperature ramp, as per FIG. 12;

FIG. 15—the performance of the coated 900 CPSI monolith is examined in atemperature programmed reactive gas soot loading;

FIG. 16—shows a TPO performed subsequent to the loading cycle of FIG.15;

FIG. 17—the effect of GHSV on B.P., CO₂ evolution and O₂ consumptionduring a soot loading in reactive gas with a simultaneous ramp from 100to 200° C. for a 900 CPSI part;

FIG. 18 a—TPO of samples ex FIG. 17—B.P. response and CO₂ evolution; and

FIG. 18 b—TPO of samples ex FIG. 17—NO and CO₂ evolution and O₂consumption.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a schematic of the synthetic gas bench (SGB) reactor inwhich the concept trials were executed. Prior to a normal experiment, amonolith core (using a flow through washcoated monolith with 400 or 900cell per square inch-CPSI) and quartz sleeve packed with quartz woolwere placed in the stainless steel reactor as shown. During the courseof the subsequent experiments the temperature, pressure drop and O₂content of the reactor were monitored using the respective probespositioned as shown in FIG. 1. Representative sampling of the gaseousreaction byproducts was performed by on-line mass spectrometry withappropriate corrections for m/z overlaps.

The test protocol employed in these studies typically consisted of twophases:

Phase 1) Soot loading cycle: In this portion of the experiment Printex Usoot analogue (obtained from Evonik Degussa) is introduced into thereactor via the use of fluidised bed system. The fluidized bed unitcontains the soot material and a flow of N₂ is based through the base ofthe bed to establish the fluid condition and thus entrain suspendedsolid material with the gas flow. The N₂/soot flow is then mixed withthe reactive gas leg and passes through the reactor, where sootdeposition in the monolith may occur. The rate of soot delivery is 0.2g/hour under typical loading conditions. In order to retain any sootpassing through the flow through monolith, i.e. determine soot ‘slip’ orlow filtration efficiency, a bed of quartz wool was packed in the outletposition of the reactor.

Phase 2) Regeneration: The sample is purged with dry N₂ and then heatedin N₂/O₂ (as a TPO or Temperature Programmed Oxidation) or reactive gasmixture (as described in FIG. 3) to 750° C. and the reactor conditionse.g. back pressure, O₂ content, temperature and off gas monitored. Notethat any soot trapped in the quartz wool is also combusted during theregeneration but only at high temperatures via the conventionalhomogeneous combustion pathway, this turn enables a determination ofsoot ‘slip’ through the monolith and thus determine the impact of celldensity on trapping efficiency. An important note is made herein in thatprior to any performance testing the various monolith cores werestabilised with respect to performance/aged, this being achieved by anin-situ thermal treatment at 750° C. for four hours.

Examples of regeneration tests for a 900 CPSI monolith coated with theactive catalyst described in Examples 1 (method of manufacture of Ag—OScomponent) and 2 (process for the production of the final coatedmonolith) are shown in FIG. 2. Herein we examine the impact of thepresence of O₂ during soot loading on the subsequent combustion(catalytic vs homogeneous) of the soot retained in the catalyst andsecondary quartz trap. The performance is broadly similar and shows thepresence of two discrete combustion events, one at ca. 250° C., ascribedto direct catalytic soot combustion arising from soot in good contactwith the active washcoat and a second event with two features at between600-700° C. from filter cake combustion and combustion of soot retainedin the quartz wool filter. This result mirrors those in SAE 2008-01-0481and re-confirms the critical importance of catalyst-soot contact fordirect oxidation to occur. A difference may be seen in O₂ consumption,as determined by the O₂ sensor, which suggests that the presence of O₂during the loading is slightly beneficial, possibly through O₂chemisorption on the surface of the soot during loading.

FIG. 3 outlines the various gas compositions employed during the loadingand regeneration trials. Hence, in subsequent figures any reference toReactive gas, for example, refers to a gas composition containing N₂,O₂, CO, NO and propene in the concentrations listed in line 3.

FIG. 4 compares the back pressure (hereafter B.P.) response for a 400CPSI (cells per square inch) flow through monolith during a three hour(10800 s) soot loading cycle as a function of either gas environmentduring loading or temperature and gas environment. The data reflect aclear difference between soot loading at 200° C. under N₂/O₂ andreactive gas mixture. In the former case there is a continuous increasein the B.P. response of the system (monolith plus quartz wool filterbed), consistent with a systematic deposition and accumulation of soot.In contrast the loading cycle at 200° C. under reactive gases shows amarkedly lower rate of B.P. increase during the accumulation cycle. Thisis consistent with a large decrease in the concentration of soot matteraccumulated within the system over time, from which it may be inferredthat there is consumption, i.e. oxidation of soot during the loadingcycle. Comparison of CO₂ evolution data during the loading cycle didshow significantly higher CO₂ for the reactive gas loading, althoughgiven the simultaneous oxidation of CO and propene this data is regardedas a partial corroboration only and subsequent TPO (FIG. 5) isconsidered more definitive. The trend of decreased B.P. increase is evenmore evident for loading cycles in reactive gas at 250 and 300° C. Thus,at 250° C. there is only a small increase in the B.P. over the cyclewhile at 300° C. the B.P. can be seen to actually decrease after theinitial loading period. Again both samples showed high levels of CO₂production during loading, consistent with continuous, direct catalytic,oxidation of soot.

Subsequent TPO, shown in FIG. 5, is consistent with the B.P. responsetrends seen during soot loading (FIG. 4). Herein TPO after the loadingcycle at 200° C. in N₂/O₂ results in a CO₂ evolution profile with threefeatures, a small oxidation feature at between 250-350° C., ascribed tocatalytic combustion of soot and two large CO₂ features at 640° C., dueto filter cake combustion, and at >700° C. ascribed to the combustion ofsoot ‘slip’ i.e. soot that passed through the monolith and was trappedin the quartz wool ‘filter’ toward the outlet of the reactor. Since thisquartz wool is located outside of the main heated zone of the furnaceany soot trapped herein is only combusted at high temperatures and henceprovides a simple diagnostic as to the extent of soot ‘slip’. Thus, inthis instance, it can be seen that at lower temperatures, and in theabsence of general combustion chemistry, there is a large ‘slip’ of sootthrough the conventional 400 CPSI part. This soot is accumulated andresults in the large B.P. increase seen in FIG. 4. This response may becontrasted with the reactive gas loading at 200° C. In this instancethere are again three main CO₂ evolution features, catalytic combustionat ca, 300° C., filter cake combustion at 650° C. and ‘slip’ combustionat ca. 710° C. However, the total CO₂ production is decreased to a largeextent, especially for the highest temperature ‘slip’ event, consistentwith increased continuous soot oxidation during the loading. Moreover,the CO₂ due to catalytic combustion is significantly increased andfilter cake CO₂ decreased, reflecting a significant enhancement ofcatalytic function under simulated exhaust conditions. These trends arefurther evident in the loading cycles at 250 and 300° C. Both showfurther decreases in total CO₂ production i.e. retained soot andespecially decreases in CO₂ due to soot ‘slip’. Hence, a comparison ofthe 300° C. reactive gas loading to the 200° C. N₂/O₂ loading shows adecrease in CO₂ of >80%, i.e. >80% of soot loaded during the cycle iscombusted via a continuous, direct catalytic, soot oxidation process.

FIG. 6 compares the O₂ concentrations at the reactor outlet during theTPO cycles described in FIG. 5. The data reflects the same trends notedabove with decreased O₂ consumption being recorded for reactive gas sootloading cycles and for soot loading cycles at 250 and 300° C. For thelatter two cycles, there is also the appearance of a feature at ca. 475°C., which does not correlate to any specific CO₂ evolution feature. Thispeak is ascribed to the desorption of NO/NO₂ from the catalyst and willbe examined in more detail in later figures (see FIGS. 9, 11, 13, 14,15, 16 and 18 b). Note, due to the positioning of the O₂ sensor at theoutlet of the monolith there is no O₂ consumption recorded for the hightemperature soot ‘slip’ event.

The impact of monolith cell density, 900 CPSI vs 400 CPSI, on the B.P.response during soot loading is recorded in FIG. 7. Comparison of theloading cycles at 200° C. show general similarities for the twosubstrates, albeit that the 900 CPSI part shows a slightly higher rateof B.P. increase during the loading cycle, consistent with the expectedlarge impact of soot accumulation in the narrower channels of thissubstrate.

A comparison of the subsequent TPO reactions after the loading cycle ofFIG. 7 is shown in FIG. 8. The data show a clear change in theeffectiveness of the technology as a function of cell density. Hence incontrast the previous data for the 400 CPSI part, the 900 CPSI substrateshows a dramatic improvement in soot filtration efficiency, with onlyvery small CO₂ evolution features seen for both filter cake and ‘slip’combustion events. Moreover, the sample also exhibits an increasedefficiency with the direct catalytic oxidation feature, hence peak CO₂production from direct catalytic oxidation is now observed at ca. 240°C. versus ca 300-310° C. for the 400 CPSI monolith. Thus by use of thehigh cell density monolith and active washcoat it is possible to achievehigh filtration efficiency, >95% based upon the total CO₂ production atT>500° C. versus the 400 CPSI monolith, and also continuous, lowtemperature, direct catalytic soot oxidation.

Comparable differences in performance with respect to O₂ consumption areobserved in FIG. 9 for the 900 CPSI vs the 400 CPSI monoliths. Ex 900CPSI O₂ consumption is predominantly seen for T<300° C., with nosignificant O₂ consumption at T>600° C. The converse is seen for the 400CPSI with a major O₂ consumption being recorded at ca. 610-620° C., fromfilter cake oxidation. Interestingly, all three samples again show anadditional feature at ca. 475° C. as per FIG. 6, associated with NOxevolution from the washcoat.

The impact of loading on temperature on subsequent TPO of accumulatedsoot is the 900 CPSI monolith result in large decreases in accumulatedsoot. Hence, while loading at 100° C. gives a peak CO₂ yield of ca. 8200counts/s, there is only 6000 c/s and 1,000 c/s for loading cycles at 150and 200° C. respectively. Moreover, for the loading cycles at 150 and200° C. there is no evidence for filter cake formation, based upon theabsence of any higher temperature CO₂ production peak. Indeedintegration of the total CO₂ evolution from the ex 200 cycle on the 900CPSI monolith versus the 200° C. N₂/O₂ cycle on the 400 CPSI monolithindicates >99% of all soot introduced during the loading cycle undergoesdirect catalytic combustion, thereby offering the potential for usage ofthe technology in a ‘real’ life application.

FIG. 11 displays the B.P. response and O₂ consumption traces associatedwith the TPO cycles described in FIG. 10. In all cases the data sets areconsistent with the observed CO₂ production profiles. Hence, in allcases, CO₂ evolution/residual soot combustion is associated with O₂consumption and with a net decrease in B.P. as the monolith channels arecleaned of the restrictive soot particles. The extent of O₂ consumptionfollows the net CO₂ production i.e. 100>150>200° C. Again, all samplesthe secondary NOx related feature at 475° C. The B.P. responses alsoappear to reflect the conditions of soot loading with the ‘relaxation’response being sharpest for the 200° C. cycle, then 150° C. and finally100° C. loading cycle, again consistent with the residual soot retentionfor the various tests.

In order to better mimic driving conditions we performed soot loadingcycles under conditions of dynamic temperature changes. Hence, FIG. 12shows an example of a temperature programmed soot loading using a 900CPSI monolith. In this test there was simultaneous soot loading cycle infull reactive gas mixture with heating of the sample from 100° C. to200° C. The data shows the expected CO (and propene) light-off curves,which were again found to be coincident with soot combustion, asreflected in the peak then decay seen for CO₂ production and O₂consumption traces. In this experiment, as in all tests performed duringthis study, there was no production of CO during the oxidation of soot(determined by analysis of the corrected mass spectrometer peak at m/z12 in which one may account for the background and dynamic contributionsof mass fragmentation from CO and CO₂). The continuous combustion ofsoot also helps to account for the overshoot seen in the bedthermocouple, which was found to be ca. 245° C. versus the set point of200° C.

FIG. 13 shows a temperature programmed reaction experiment performedafter the temperature programmed reaction soot loading in FIG. 12. Theprotocol for this test entailed cooling the sample in-situ to 100° C. inflowing N₂, after the soot loading cycle was completed, uponstabilisation at 100° C., the full reactive gas mixture was thenreintroduced, and the sample heated to 750° C., per standard method. Thedata shows the expected light-off of CO (propene also undergoeslight-off but the signal is omitted for clarity with CO, NO and NO₂traces) as evidenced by the responses in CO, CO₂ and also the O₂ sensor.Interestingly, there is again a peak of CO₂ production at ca. 225° C.,and then a decrease, this feature is ascribed to the combustion of theresidual soot retained on the part. During this combustion event thereis no significant change in the B.P. of the sample, suggesting theretained soot is at such a low level as to not result in any meaningfulcontribution to system backpressure. Finally, there is a very small CO₂evolution at 475° C., this latter feature is coincident with theapparent O₂ consumption event noted in previous tests (see FIGS. 9 and11) but also with a NOx (NO and NO₂) desorption event. This event isattributed to the intrinsic NOx scavenging and release properties of theAg—OS material, as described in SAE 2008-01-0481, application Ser. Nos.12/363,310 and 12/363,329. Thus, during the loading cycle and in thesubsequent temperature programmed reaction, any NO₂ that is generatedwhich would normally result in ‘de-coupling’ of catalyst-soot contact,is trapped on the highly dispersed Ag centres and retained to hightemperatures where it is released in the plume observed. The plume ofdesorbed NOx then may react with any traces of soot remaining on thepart, particularly any species that are spatially distant from thecatalyst surface i.e. with ‘poor’ contact.

To further examine the impact of temperature during a reactive sootloading cycle additional tests were performed. FIG. 14 shows the TPOresults for the 900 CPSI monolith after a soot loading with reactive gasand temperature ramp, as per FIG. 12. Employing the TPO protocol ratherthan the full reactive gas mix simplifies the chemistries and resulttraces. Thus, in the TPO protocol there are no light-off features butrather a series of peaks due to the various phenomena occurring over thecatalyst versus temperature. Firstly there is a CO₂ production peak,attributed to the combustion of residual retained soot. This peak iscentred at 300° C., ca. 75° C. higher than in the temperature programmedreaction case. This reflects the important contribution of theexothermicity of CO and HC light-off in facilitating lower temperaturesoot oxidation. Hence, in the reactive gas temperature ramp, as the COand HC begin to combust they generate a thermal bloom with the monolithwhich is sufficient to overcome the activation energy barrier for theinitiation of soot oxidation. Then once the combustion of soot isinitiated, a further exotherm is generated and the resulting thermal‘cascade’ is sufficient to result in very high soot conversion rates,this process is the related of the method for lower temperature sootoxidation as described in US 2005/0282698 A1. The soot oxidation eventin this instance is correlated to a very small B.P. ‘relaxation’. Thesample is also seen to desorb water, this release being associated withdesorption of combustion by-products from HC oxidation. Finally, at 475°C. one again sees the NOx desorption/apparent O₂ consumption eventassociated with the Ag—OS scavenging function, however in this instancethere does not appear to be any significant associated CO₂ productionfrom the combustion of trace soot in poor contact. However, what isclear is that under the loading conditions employed there isconsistently high activity towards direct soot oxidation resulting invery low levels of residual soot remaining on the 900 CPSI monolith,again confirming the potential for the approach for continuous, directcatalytic, soot oxidation.

In FIG. 15 the performance of the coated 900 CPSI monolith is examinedin a temperature programmed reactive gas soot loading. In this instancethe maximum temperature employed was 500° C. (ramping from 50° C.). Inthe case the CO (and HC) light-off traces are very clearly represented,as is the associated O₂ consumption. Again the CO₂ evolution trace showsan increase to a peak at ca. 250° C. before decreasing to a steady statevalue. This is again consistent with the active catalytic combustion ofretained soot. Hence, in this and all other regards the performancetrends replicate previous findings, including the NOxscavenging/apparent O₂ consumption at ca. 475° C.

FIG. 16 shows a TPO performed subsequent to the loading cycle of FIG.15. Herein there are no significant reaction or desorption eventsevident. In particular, there is no additional CO₂ production, no hightemperature soot ‘slip’ phenomenon, i.e. the data is consistent withcomplete conversion of any soot loaded during the loading cycle, furtherconfirming the high effectiveness of the technology.

Next the impact of Gas Hourly Space Velocity on performance wasexamined. Hence, FIG. 17 contrasts reactive gas loading cycles, withtemperature ramp (100-200° C.) under the standard GHSV of 15000 h⁻¹versus a GHSV of 25000 h⁻¹ (versus monolith volume). It is emphasised atthis point that the soot delivery rate in both tests as determined bythe flow rate through the fluidised bed was constant in both cases andthe increase in GHSV was achieved by increasing the flow rates of thevarious gases within the reactive gas manifold. Analysis of thesubsequent data from both tests show comparable response with responseto gas phase chemistry, with CO (and HC) light-off being unaffected, asevidenced by the comparable CO₂ responses. There is an offset in the O₂sensor values, possibly due to the total increased flow employed in thehigh GHSV test, but again the dynamic responses are identical. Aftercompletion of light-off there is some difference in the CO₂ responses,with the low GHSV test showing higher net CO₂. Coincident to this, theB.P. response of the high GHSV test shows a steady increase, this isascribed to a combination of the higher net static back pressureobserved due to the higher flow rate but also to an increase in the netrate of soot accumulation during the test. This raises questionsregarding location of soot, i.e. is the B.P. increase due to soot ‘slip’or is the soot still retained on the part, and also what is the maximumeffective rate of soot deposition that may be employed and still achievehigh continuous soot combustion rates.

In FIG. 18 a/b these questions are answered. Herein the subsequent TPOcycles loading cycles. The data show only CO₂ production at lowertemperatures with a peak at ca. 300° C. Hence even under the conditionsof higher flow there was no soot slip i.e. all soot introduced wasretained with the monolith. The decrease in rate of soot oxidation isthus ascribed to an exothermal effect with the increasing flow ratethrough the part during loading resulting in a net ‘dilution’ of theexotherm cascade which is believed to be critical for the propagation ofsoot burn. However, as indicated the ‘excess’ soot generated due to thisprocess is merely retained unreacted on the part, thus in the subsequentTPO the soot oxidation follows the same profile as the lower GHSV caseand there is simply an increase in the net CO₂ production. This is alsoreflected in the B.P. responses with the sample loaded under high GHSVshowing a more rapid and larger B.P. ‘relaxation’ than the sample loadedunder lower GHSV. Similarly the NOx evolution response is larger for thehigh GHSV sample, reflecting the higher mass fraction of NOx exposureduring the test. This in turn results in the small differences inapparent O₂ consumption, as recorded by the O₂ sensor. Thus, to concludewhile there is an impact of GHSV on activity, the impact is notcatastrophic and the monolith retains its ability to either combust ortrap all soot at lower temperatures and then to facilitate itscombustion at temperatures still within the normal operating window of aconventional vehicle, i.e. the system still provides effective sootfiltration and combustion without recourse to conventional activeregeneration strategy.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and apparatus for thecontinuous/semi-continuous direct catalytic, oxidation of dieselparticulate matter by the combination of base metal modified OxygenStorage (OS) materials in association with turbulent flow/high celldensity flow through monoliths. The particular combination of the basemetal modified OS direct soot oxidation catalyst with the flow throughmonolith provides a synergy which enables high conversion of particulatematter without the backpressure penalty introduced by the conventionalDPF. It is believed that the synergy arises from the ability of theactive OS material to combust soot at lower temperatures than inconventional systems, which in turn is facilitated by the decreasedthermal mass of the conventional substrate, with the latter stillproviding sufficient geometric surface area for soot deposition andreaction. Large improvements in lower temperature activity can beobtained in marked contrast to the conventional wall flow DPF whereinlarge thermal mass of the substrate, particularly for SiC DPF, inhibitsinitiation and especially propagation of soot combustion.

The present invention represents a significant advance in thedevelopment of a method and apparatus for the (semi) continuous, directcatalytic, oxidation of diesel particulate matter may be realized by thecombination of base metal modified Oxygen Storage (OS) materials with aconventional flow substrate. The substrate is selected from a range ofceramic or metallic technologies upon which the active washcoat isdisposed. Such substrates can be metallic parts, ceramic or metal foams.

More particularly and in a further aspect, the present invention relatesto a synergistic combination of a catalyst and a substrate for thefiltration and continuous, direct catalytic, oxidation of dieselparticulate matter at low temperatures. The catalyst comprisescatalytically active precious metal (Pt, Pd, Rh or combinationsthereof), a host cerium-based solid solution which is a substantiallyphase pure cubic fluorite (as determined by x-ray diffraction method) ofthe CeZrOx type which is well known in the art and a refractory oxidesupport, e.g. (γ)Al₂O₃, ZrO₂ or other known oxide support. The CeZrOx isfurther modified by the incorporation of an active base metal, e.g. Ag,Cu etc. as disclosed in application Ser. No. 12/363,310. The catalystfurther comprises a monolith substrate, of conventional design, whereinthe monolith is an inert ceramic or metal substrate upon which theactive catalyst formulation/washcoat is disposed. The monolith substrateis further characterised by a high cell density, i.e. a large number ofactive channels per unit area, for effective synergy a value of >600cells per square inch. In the case of a metallic substrate the activewashcoat may be applied to the perforated, punched and embossed metalfoils (e.g. TS, LS, PE and MX type systems; see for example U.S. Pat.No. 6,689,327) with beneficial effect.

The combined active washcoat and monolith system may be applied to thechallenge of particulate emission control catalysts for diesel (or otherfuel lean) or potential gasoline (stoichiometric) application. Theparticular example described herein is for the application of thesematerials in the area of continuous, direct catalytic oxidation ofdiesel particulate matter upon its interaction with the high celldensity substrate. These benefits arise in this application due to theaforementioned synergies arising from the high cell density monolith andthe new generation of modified OS materials. The latter has beenpreviously been demonstrated as having benefits in affecting eitherlower temperature regeneration/oxidation of soot or an increasedregeneration efficiency at a lower temperature as compared tonon-modified OS materials (application Ser. Nos. 12/363,310 and12/363,329). Now in combination with a conventional flow monolith ofappropriate architecture, it becomes possible to realise a completelypassive particulate control catalyst.

It should be further noted that the terms “first”, “second” and the likeherein do not denote any order of importance, but rather are used todistinguish one element from another, and the terms “a” and “an” hereindo not denote a limitation of quantity, but rather denote the presenceof at least one of the referenced items. Furthermore, all rangesdisclosed herein are inclusive and combinable (e.g., ranges of “up toabout 25 weight percent (wt. %), with about 5 wt. % to about 20 wt. %desired, and about 10 wt. % to about 15 wt. % more desired” is inclusiveof the endpoints and all intermediate values of the ranges, e.g. “about5 wt. % to about 25 wt. %, about 5 wt. % to about 15 wt. %” etc.

The details regarding the synthesis, characterization and preferredcompositions, structures, dopant levels etc for the Cerium-containingmixed oxide/solid solution material are detailed in Ser. Nos. 12/363,310and 12/363,329. Preferably, the solid solution contains a cationiclattice with a single-phase, as determined by standard x-ray diffractionmethod. More preferably this single-phase is a cubic structure, with acubic fluorite structure being most preferred. Additionally, it is notedthat the doping process may be performed without formation of anadditional bulk phase, as determined by XRD. In various embodiments, theOS material may include those OS materials disclosed in U.S. Pat. Nos.6,585,944; 6,468,941; 6,387,338 and 6,605,264 which are hereinincorporated by reference in their entirety. However, the flexibility ofthe basic exchange provides for the modification of all current knowncerium oxide and Ce—Zr-based solid solution materials to be thuslymodified and enhanced.

As indicated, the OS materials modified by the doping method shallpreferably be characterized by a substantially cubic fluorite structure,as determined by conventional XRD methods. The percentage of the OSmaterial having the cubic structure, both prior and post exchange, ispreferably greater than about 95%, with greater than about 99% typical,and essentially 100% cubic structure generally obtained (i.e. animmeasurable amount of tetragonal phase based upon current measurementtechnology). The exchanged OS material is further characterized in thatit possess large improvements in durable redox activity with respect tofacile oxygen storage and increased release capacity e.g. as determinedby H₂ Temperature Programmed Reduction (TPR) method. Thus, for Cuexchanged solid solutions, for example, the reduction of Ce+Cu isobserved to occur at a temperature of about 300 to about 350° C. lowerthan would occur in the absence of the Cu dopant (see application Ser.No. 12/363,310).

In an exemplary embodiment, an active soot oxidation catalyst comprisinga precious group metal or metals (Pt, Pd, Rh and combinations thereof),a base metal doped cerium-oxide containing solid solution and arefractory oxide carrier all of which together are employed as acoating, e.g., disposed on/in an inert substrate or carrier, thesubstrate or carrier being characterized by a high number of channels orcells per unit area or by the its ability to introduce turbulent flowdue to the construction of its internal flow channels. Exhaust gastreatment devices can generally comprise housing or canister componentsthat can be easily attached to an exhaust gas conduit and comprise asubstrate for treating exhaust gases. The housing components cancomprise an outer “shell”, which can be capped on either end withfunnel-shaped ‘end-cones’ or flat ‘end-plates’, which can comprise‘snorkels’ that allow for easy assembly to an exhaust conduit. Housingcomponents can be fabricated of any materials capable of withstandingthe temperatures, corrosion, and wear encountered during the operationof the exhaust gas treatment device, such as, but not limited to,ferrous metals or terrific stainless steels (e.g., martensitic,terrific, and austenitic stainless materials, and the like).

Disposed within the shell can be a retention material (“mat” or“matting”), which is capable of supporting a substrate, insulating theshell from the high operating temperatures of the substrate, providingsubstrate retention by applying compressive radial forces about it, andproviding the substrate with impact protection. The matting is typicallyconcentrically disposed around the substrate forming a substrate/matsub-assembly.

Various materials can be employed for the matting and the insulation.These materials can exist in the form of a mat, fibres, preforms, or thelike, and comprise materials such as, but not limited to, intumescentmaterials (e.g., a material that comprises vermiculite component, i.e.,a component that expands upon the application of heat), non-intumescentmaterials, ceramic materials (e.g., ceramic fibers), organic binders,inorganic binders, and the like, as well as combinations comprising atleast one of the foregoing materials. Non-intumescent materials includematerials such as those sold under the trademarks “NEXTEL” and “INTERAM1101HT” by the “3M” Company, Minneapolis, Minn., or those sold under thetrademark, “FIBERFRAX” and “CC-MAX” by the Unifrax Co., Niagara Falls,N.Y., and the like. Intumescent materials include materials sold underthe trademark “INTERAM” by the “3M” Company, Minneapolis, Minn., as wellas those intumescent materials which are also sold under theaforementioned “FIBERFRAX” trademark.

Housings as described above are well known and understood by thoseskilled in the art.

The substrates or carrier employed in this invention can comprise anymaterial designed for use in a spark ignition or diesel engineenvironment having the following characteristics in addition to the highcell density/turbulent flow requirement stated previously: (1)capability of operating at temperatures up to about 600° C. and up toabout 1,000° C. for some applications, depending upon the device'slocation within the exhaust system (e.g., manifold mounted, closecoupled, or underfloor) and the type of system (e.g., gasoline ordiesel); (2) capability of withstanding exposure to hydrocarbons,nitrogen oxides, carbon monoxide, particulate matter e.g. soot and thelike, CO₂, and/or sulfur; and (3) have sufficient surface area andstructural integrity to support a catalyst, if desired. These materialsshould be inert under the conditions imposed on them when in use. Somepossible materials include cordierite, silicon carbide, metal, metaloxides; e.g. alumina and the like, glasses and the like, and mixturescomprising at least one of the foregoing materials. Some suitable inertceramic materials include ‘Honey Ceram’, commercially available fromNGK-Locke, Inc, Southfield, Mich., and ‘Celcor’, commercially availablefrom Corning, Inc., Corning, N.Y. These materials can be in the form offoils, perform, mat, fibrous material, monoliths e.g. a honeycombstructure, and the like, other porous structures e.g., porous glasses,sponges, foams, pellets, particles, molecular sieves, and the like(depending upon the device), and combinations comprising at least one ofthe foregoing materials and forms, e.g., metallic foils, open porealumina sponges and porous ultra-low expansion glasses. Furthermore,these substrates can be coated with oxides and/or hexaaluminates, e.g.stainless steel foil coated with a hexa-aluminate scale.

Although the substrate can have any size or geometry, within thepreviously defined limits, the size and geometry are preferably chosento optimize surface area in the given exhaust gas emission controldevice design parameters. Typically, the substrate has a honeycombgeometry, with the combs through-channel having any multi-sided orrounded shape, with substantially square, triangular, pentagonal,hexagonal, heptagonal, or octagonal or similar geometries preferred dueto ease of manufacturing and increased surface area.

The exhaust gas treatment devices can be assembled utilizing variousmethods. Three such methods are the stuffing, clamshell, and tourniquetassembly methods. The stuffing method generally comprises pre-assemblingthe matting around the substrate and pushing, or stuffing, the assemblyinto the shell through a stuffing cone. The stuffing cone serves as anassembly tool that is capable of attaching to one end of the shell.Where attached, the shell and stuffing cone have the samecross-sectional geometry, and along the stuffing cone's length, thecross-sectional geometry gradually tapers to a larger cross-sectionalgeometry. Through this larger end, the substrate/mat sub-assembly can beadvanced which compresses the matting around the substrate as theassembly advances through the stuffing cone's taper and is eventuallypushed into the shell.

Exhaust gas treatment devices comprising the doped solid solutions canbe employed in exhaust gas treatment systems to provide both an activesoot combustion catalyst but also a NOx adsorption function, and thusspecifically reduce a concentration of undesirable constituents in theexhaust gas stream. For example, as discussed above, an exemplarycatalyst system can be formed utilizing the doped OS as a catalystcomponent, wherein the catalyst system is disposed on a substrate, whichis then disposed within a housing. Disposing the substrate to an exhaustgas stream can then provide at least a NOx storage function, anddesirably even reduce the concentration of at least one undesirableconstituent contained therein.

According to one embodiment of the present invention, the catalyst doesnot conform the standard architecture of a CDPF or Diesel NOxParticulate Trap and hence does not comprise a porous substrate havingalternating channels. Rather the preferred configuration of the catalystis as a conventional ‘flow through’ monolith, of high unit cell countper unit area, upon which is disposed the active catalyst washcoat. Thecombination of the active washcoat with the high internal surface areaand turbulent deposition mechanism is sufficient to facilitate retentionand continuous particulate oxidation under conventional operatingtemperatures and flows of a diesel/compression ignition vehicle.

EXAMPLES

The benefits obtained by the active washcoat employing the doped Ceriumcontaining oxide and high cell density monolith are clearly evident inFIGS. 4-18 b, wherein the benefit of enhanced redox performance of thedoped OS, in combination with an appropriate substrate result in highrates of direct soot combustion under conditions appropriate forvehicular application. It should be stressed that the redox promotionobtained by base metal doping is observed for both a range of cationicdopants and a range of OS compositions, and the data included herein forthe 2Ag—OS is merely a representative example.

The data herein reflect a systematic study of the various parametersconsidered relevant to achieve the desired aim of continuous, lowtemperature, direct catalytic soot oxidation. The impacts of theseparameters on performance are summarized, with reference to specificcase data as follows:

a) Reactivity of soot: The reactivity of soot e.g. soluble organicfragment has been shown to play a large role in determining thereactivity of soot and thus the effective performance of soot oxidationcatalysts (Atmos Env, vol. 15 (1), 1981, 91-94, SAE paper 2008-01-0481,App Catal B, vol. 75 (1-2), 2007, p 11-16, etc.). Indeed, comparison ofconventional soot TGA shows an increase in Tmax (temperature for maximumrate of soot combustion) of ca. 50° C. for Printex U cf. ‘real’ dieselsoot collected from a vehicle (SAE 2008-01-0481). Thus, in this studyPrintex U soot analogue was employed to specifically remove thisvariable from any discussion. Hence, all particulate matter combustedduring these tests may be considered to be equivalent in reactivity andthus present no inherent bias in any given data set. Moreover, theoxidation of the Printex material may be considered to be a ‘worst case’scenario i.e. its oxidation is representative of combustion of very‘dry’, refractory carbonaceous material high in graphitic content andlow in SOF. Thus, the promising data herein reflect a true performanceadvantage of interest to real world applications.

b) Gas environment during soot accumulation: There is a clear impact ofreactive gas chemistry on catalyst performance both during the loadingcycle as shown in FIGS. 2, 4, 5 and 6 but also the nature of the gasatmosphere can be seen to impact the regeneration, as is evident fromcontrasting TPO versus temperature programmed reaction burn outprotocols (FIGS. 5, 8, 10, 13, 14, 16 and 18 a/b). This impact isattributed to a combination of heat transfer and catalyst activation.One heat transfer component arises due to the external heating of theactive catalyst arising from the combustion of the significant levels offuel components within the reactive gas mixture, principally CO and HC.This energy is retained within the washcoat, resulting in the hotterthan expected bed temperatures observed, and thus helping to overcomethe activation energy barrier to catalytic soot oxidation. A secondcombined heat transfer and catalyst activation component is arises fromthe activation of the redox oxide arising from its participation in theCO oxidation process. It has been shown that the doped cerium oxides areeffective oxidation catalysts, even in the absence of PGM, and canfacilitate CO oxidation at low temperatures (DP-316440). In doing so thecatalyst O ion transport function is activated, and energy released atthe active site of CO oxidation. The subsequent re-oxidation of thedepleted oxygen results in a further exotherm, distributed throughoutthe entire structure of the OS, in a sense further priming the OS toinitiate soot oxidation. This mechanism forms part of the basis of US2005/0282698 A1, wherein a fuller explanation may be found.

c) Static temperature effect during soot accumulation: Obviously thermalenergy/temperature is required to overcome the activation energy barrierfor catalytic soot combustion. Hence, with increasing inlet temperature,there is a concurrent increase in rate of catalytic oxidation and hencedecreases in soot slip and mass of retained soot on the monolithirrespective of all other factors (FIGS. 4-18 b).

d) The role of cell density of the monolith: This is a key factor forthe invention, with the use of higher cell density/increased cell countper unit area, resulting in a large enhancement of catalyst performance(FIGS. 7-9). Thus by merely substituting the 400 CPSI monolith with, the900 CPSI one results in a dramatic improvement in soot filtrationefficiency (>95% based upon the total CO₂ at T>500° C. vs 400 CPSI), theability to circumvent soot ‘slip’ through the monolith i.e. no hightemperature CO₂ production due to soot passing through the monolith andbeing retained in the quartz wool filter, and also a small decrease intemperature required for soot combustion (which is attributed to thehigher effectiveness of the oxidation washcoat). The impact of celldensity also has a positive synergy with temperature with larger netperformance gains being observed for the 900 CPSI monolith for highertemperature than for the 400 CPSI system.

e) The role of NOx on catalyst performance: The ability of the metaldoped OS material to scavenge NOx at low temperatures and release theretained species at higher temperatures (see FIGS. 13, 14, 15 and 18 b)is of particular importance. This capability effectively disables the‘de-coupling’ mechanism of related to NO₂ mediated soot oxidation, whichhas been shown to destroy the intimate contact between catalyst and sootrequired for direct catalyzed soot oxidation (see SAE paper2008-01-0481, U.S. patent application Ser. No. 12/363,329). Indeed byuse of the metal doped OS material it appears that one can, in selectedcases, also employ the NOx desorption plume beneficially to remove traceparticulate matter that is in poor contact i.e. spatiallydiscrete/removed from the active catalyst e.g. see FIG. 13. However, itmust be stressed this is not the primary catalytic process responsiblefor the high activity seen at low temperatures but is rather anadditional minor benefit.

g) Dynamic temperature effect during soot accumulation: Comparison ofstatic temperature soot loading and regeneration cycles (FIGS. 4, 5, 7,8 and 11) versus loading and regeneration cycles with dynamictemperature change i.e. temperature ramp (FIGS. 12, 13, 14 and 15)illustrate a further manifestation of energy with the process. Thus itmay be seen that increases in temperature during the soot accumulationprocess, combined with the specific exotherm associated Light-off of COand HC, result in a further improvement in performance compared to asimple static temperature loading. This is ascribed to the combinationof exothermal effects, which we have dubbed thermal bloom propagation,described previously and explained in more detail in US 2005/0282698 A1.

h) The effect of GHSV and flow velocity: Clearly, the residence time ofthe particulate matter within the monolith is an important factor. Thus,the longer the particulate reside within the monolith channels, thegreater is the probability of interaction with the active washcoatcoated on the walls and, hence, the higher probability of retention andreaction. Moreover, since the particulates are entrained by the flowi.e. derive their kinetic energy from Brownian collisions, at higherflow rates, the velocity of the particulates are higher. This bothdecreases residence time within the monolith but also provides a forcedriving the following regime to be more laminar and less turbulent,thereby decreasing the possibility of particulate to wall interaction.These hypotheses are consistent with data in FIGS. 17, 18 a and 18 b. Inaddition at higher flow velocities there is an increase in energytransport out of the monolith, i.e. the local exotherms are diminisheddue to increased Brownian collisions transferring kinetic energy tomolecules leaving the flow channel. Hence, at higher GHSV the rate ofsoot oxidation is somewhat decreased and results in increased formationof retained soot species. However, under the conditions examined, theincreased flow was not enough to result in soot ‘slip’, preventextensive direct catalytic oxidation or indeed prevent completeregeneration in the subsequent burn out cycle. Finally, it should benoted that during the burn out cycle the temperatures required forcombustion of the retained soot mass fraction was still significantlylower than the >600° C. employed in conventional DPF activeregeneration. Indeed the temperatures required were still only of theorder of 300-330° C., i.e. temperatures easily within the normaloperational window of a diesel vehicle. Hence, the concept of directcatalytic soot oxidation is applicable to vehicular application.

EXAMPLES

The procedure for making 100 grams of 2% Ag(NH₃)₂ OS, as employed in thetest technology is as follows:

1. Weigh 100 g of OS, correct for moisture content (ca. 1.5% water).

2. Weigh 3.15 g of silver nitrate crystals. One must compensate for thepercentage of metal in the nitrate salt or solution used. Silver nitrateis 63.52% silver.

3. Dissolve silver nitrate in 50 g deionised water. The amount of waterused is determined by the water adsorption capacity of the mixed oxideused. This is generally between 0.5 and 0.5 g water per gram mixedoxide.

4. Add concentrated NH₄OHaq (30% ammonia) to the silver nitratesolution, dropwise, until a clear silver diamine solution is obtained.Solution will first turn brown-black, then clear upon excess addition ofammonium hydroxide.

5. Add silver diamine solution to mixed oxide powder. Mix thoroughly toproduce homogeneous and even-colored moist powder.

6. Allow powder to rest at room temperature for one hour.

7. Dry in oven at ˜110° C. for ca. two hours or until dry.

8. Calcine in furnace at 540° C. for four hours in air.

OS=40% CeO₂; 50% ZrO₂/HfO₂; 5% La₂O₃; 5% Pr₆O₁₁

The procedure for producing the active washcoat and producing the 400and 900 CPSI parts tested in this study is as follows: Slowly addalumina with milling to a d₅₀ of 7 microns (±1), d₉₀=20-25 and 100%pass<60 microns. Adjust pH to 3.0-3.5 and specific gravity to allow onepass coating then coat monolith in one pass and calcine at temperatures≧540 C for ≧one hour. Next slurry required 2Ag—OS in DI water, mill atthe natural pH of the material to a d₅₀ of 2±0.3, d₉₀ of <10 microns and100 pass<30 microns. Prevent pH decreasing below 4 by addition of base.Next pre-mix pt nitrate and pd nitrate solutions for 15 minutes. To thismixture add dilute sugar solution and mix for a minimum of 30 minutes;add to Ag—OS slurry within 60 minutes of initial mixing to avoidprecipitation of metal. Add PGM sugar solution dropwise to Ag—OS slurryvortex. Prior to addition slurry must be at a pH of 5.5-6.0 and duringmetal addition, prevent slurry from going to pH values below 4.0 withthe judicious use of base. Stir two hours to allow full chemisorption.Adjust pH and specific gravity to allow one pass coating then coatmonolith in 1 pass and calcine at temperatures ≧540° C. for ≧one hour.

1. A catalyst system for the direct catalytic oxidation of particulatematter in the off-gas of an internal combustion engine wherein thesystem comprises a standard flow through monolith device, upon which iscoated an active oxidation catalyst formulation for the direct, lowtemperature oxidation of aforementioned particulate matter, with theactive catalyst containing an active redox oxide disposed therein. 2.The catalyst system of claim 1, wherein the monolith is a flow throughmonolith with >900 cells per square inch.
 3. The catalyst system ofclaim 1, wherein the monolith is a flow through monolith with >600 cellsper square inch.
 4. The catalyst system of claim 1, wherein the monolithis a flow through monolith with >400 cells per square inch.
 5. Thecatalyst system of claim 1, wherein the monolith is a metal monolithcapable of introducing turbulent flow in the exhaust stream.
 6. Thecatalyst system of claim 1, wherein the monolith is a metal or ceramicfoam presenting a flow path of highly tortuous nature.
 7. The catalystsystem of claim 1, wherein the catalyst system is a refractory oxide. 8.The catalyst system of claim 1, wherein the catalyst system containscerium.
 9. The catalyst system of claim 1, wherein the oxide is a ceriumoxide in the form of a solid solution of cerium and zirconium oxide(Ce—Zr oxide).
 10. The redox active oxide of claim 1, wherein the oxideis a cerium oxide in the from of a Ce—Zr oxide solid solution that issubstantially phase pure cubic fluorite solid solution (as determined byconventional XRD method) with oxygen ion conducting properties andcomprises a. up to about 95% zirconium b. up to about 95% cerium c. upto about 20% of a stabiliser selected from the group consisting of rareearths, yttrium and mixtures thereof.
 11. The catalyst system of claim1, wherein the catalyst system is a substantially phase pure cubicfluorite solid solution additionally modified by the introduction of oneor more base metal dopant species selected from the group consisting ofa transition metal, an alkali metal, an alkaline earth metal and a groupIIIb metal.
 12. The catalyst system of claim 11, wherein the redox oxideis a base metal doped cerium containing cubic fluorite solid solutionproduced by contacting redox active material with a precursor solutionof dissolved cations under conditions of high ph/low hydronium ion(H₃O⁺)/low proton (H⁺) content.
 13. The catalyst system of claim 12,wherein the base metal is introduced into the redox active oxide bymeans of an ammonium hydroxide/ammoniacal complex of the metal cation.14. The catalyst system of claim 12, wherein the base metal isintroduced into the redox oxide by means of an organic amine complex ofthe metal cation.
 15. The catalyst system of claim 12, wherein the basemetal is introduced into the redox oxide by means of a hydroxidecompound of the metal cation.
 16. The catalyst system of claim 12,wherein the concentration of metal species introduced is about 0.01weight % to about 10 weight %.
 17. The catalyst system of claim 16,wherein the concentration of metal species introduced is most preferably0.1 wt % to about 2.5 wt %
 18. The catalyst system of claim 12, whereinthe base metal doped solid solution contains metal at high levels ofdispersion such that phase analysis by conventional XRD methods retainsa substantially phase pure cubic fluorite phase (>95%), with bulk metaloxide dopant phase being recorded at <5% and dopant metal oxide particlesize, as determined by line-broadening/Scherrer equation method, isabout 30 A to about 100 A.
 19. The catalyst system of claim 12, whereinthe base metal doped solid solution contains metal at high levels ofdispersion such that phase analysis by XRD reveals the promoted materialmaintains at least 95% cubic fluorite phase after hydrothermal oxidisingaging at 1100° C.
 20. The catalyst system of claim 12, wherein the basemetal doped solid solution contains metal at high levels of dispersionsuch that phase analysis by XRD reveals the promoted material maintainsat least 99% cubic fluorite phase after hydrothermal oxidizing aging at1100° C.
 21. A device for the direct catalytic oxidation of sootcomprising the catalyst system of claim 1, and a housing wherein thetemperature of required for soot oxidation is about 100 to about 650° C.22. A device for the direct catalytic oxidation of soot comprising thecatalyst system of claim 1, and a housing wherein the temperature ofrequired for soot oxidation is about 200 to about 400° C.
 23. A devicefor the direct catalytic oxidation of soot comprising the catalystsystem of claim 1, and a housing wherein continuous soot oxidationoccurs for temperatures of about 100 to about 650° C.
 24. A catalyticsystem for the direct catalytic oxidation of soot according to claim 1,wherein the catalyst system is free of a platinum group metal.
 25. Thecatalyst system for the direct catalytic oxidation of soot according toclaim 1, further comprising a platinum group metal.
 26. The catalystsystem for the direct catalytic oxidation of soot according to claim 25,wherein the platinum group metal is selected from the group consistingof platinum, palladium, rhodium and mixtures thereof.
 27. The catalystsystem for the direct catalytic oxidation of soot according to claim 25,further comprising a catalytically active washcoat disposed upon themonolith as a single layer washcoat which additionally contains Al₂O₃,modified Al₂O₃, SiO₂, ZrO₂, or combinations thereof or other suitablerefractory oxide as an additional support or binding agent.
 28. Thecatalyst system for the direct catalytic oxidation of soot according toclaim 25, further comprising a catalytically active washcoat disposedupon the monolith in two or more layers with a first layer containingsubstantially Al₂O₃, modified Al₂O₃, SiO₂, ZrO₂, combinations thereof orother suitable refractory oxide as a support or binding agent and asecond layer comprising the active oxidation catalyst formulation,including a base metal doped mixed oxide.
 29. A method of treatingexhaust gas comprising passing an exhaust gas over the catalyst systemof claim 1.